This invention relates to steam generation by direct contact between high temperature gases produced by combustion of gaseous hydrocarbon fuels such as natural gas and an oxidizer such as compressed air and water. The invention also provides a method of igniting a high pressure gaseous fuel/oxidizer burner uitlizing a pyrophoric compound with alternate combustor configurations. The disclosed steam generator is of improved construction and utilizes fluid injection for varying combustion processes in situ, resulting in substantially increased operational periods when generating steam for tertiary oil recovery in downhole combustion.
Techniques for thermal recovery of oil have been known for a substantial period of time. Although above ground steam generation and introduction into wells, also known as "steam drive", is in common use, the technique suffers from substantial limitations, particularly in deeper wells. Included in these limitations is the loss of heat due to long flow paths from the steam generator to the oil bearing strata or sands containing oil requiring steam injection for recovery.
Direct fired downhole steam generation such as disclosed in U.S. Pat. No. 2,548,606, (known as DFDSG) overcomes many of the above mentioned difficulties. In U.S. Pat. No. 2,548,606, hereby incorporated by reference is typical of conventional DFDSG's. However, the system disclosed typically displays substantial operating difficulties, culminating in short burner runs and substantially reduced "recovery."
High pressure combustion and steam generation encountered in downhole recovery also presents additional difficulties, including ignition, and corrosive deterioration of the burner assembly. Additional approaches to downhole recovery are disclosed in U.S. Pat. No. 2,839,141. This approach generates steam at the surface.
Known direct fired downhole steam generators have encountered certain operating difficulties resulting in reduced operating times, and relatively short equipment life, particularly that of the combustor. An example of the substantially limited life of direct fired downhole steam generators (DFDSG) is contained in reports published by the Sandia National Laboratories, working under contract to the United States Department of Energy. These reports, titled "Air/Diesel Steam Generator Fuel Test Interim Report" dated June 10, 1982, and "Oxygen/Diesel Steam Generator Field Test Interim Report" dated June 10, 1982, and Project Deep Steam Quarterly Reports Oct. 1, 1981-Mar. 31, 1982, indicate the deterioration of a DFDSG. More particularly, damage to areas adjacent to the combustor can and occurrences of "map" cracking are shown as examples of the deterioration of known generator designs. A further difficulty pointing up the lack of reliability when utilizing glow plugs for ignition is also indicated in these reports. Igniting the burner of a "downhole" generator in situ, as indicated above, is therefore an additional and substantial problem with known equipment.
U.S. Pat. No. 3,456,721 discloses a DFDSG unit employing a ceramic liner and conventional electrical ignition. In situ life of the electrical ignitor and associated difficulties are limited due to the high downhole fuel/oxidizer pressures resulting in limited actual combustion time downhole. Life of the ceramic liner disclosed is also limited in the downhole environment.
As indicated above, ignition of the combustor utilized in high pressure downhole steam generators is difficult and complicated. Difficulties arise since the energy required to ignite even stochiometric mixtures is great when both fuel and oxidizer mix at high pressures and flow rates. The conventional spark ignition at high pressures is impractical due to the distances from a power source, and the large sparking potentials required.
Use of resistance heaters known as "glow plugs" provides the bulk of presently used ignitors and avoids certain of the problems encountered. However, subsequent high temperature combustion after ignition greatly reduces usable life of these units. Therefore, in order to provide economic and long term combustion "runs" of a downhole steam generator, a non-deteriorating source of ignition energy as disclosed herein is a substantial advance in the ignition art.
U.S. Pat. No. 2,941,595 discloses a method and structure for spontaneous ignition of a burner utilizing premixed gaseous fuel/oxidizer. However, the ignitor disclosed is a solid metal phosphide, requiring contact with water in order to produce temperatures sufficient for ignition of the fuel and air mixture. Utilizing a solid ignitor also requires use of an additional fluid and makes positioning of the igniting material difficult to introduce and/or control. In the pyrophoric technique disclosed, these shortcomings are overcome and precise introduction and control of an igniting mixture is provided.
It is therefore an object of this invention to provide a direct fired downhole steam generator utilizing designs which minimize thermal and/or mechanical stresses.
It is a further object of this invention to provide a direct fired downhole steam generator where combustion pulsations are controlled, thereby minimizing structural fatigue of the burner components without sacrificing burner output or efficiency.
It is an additional object of this invention to provide a method of controlling a direct fired downhole steam generator through the use of injected fluids such as water, in order to modify the ongoing combustion process.
It is a further object of this invention to provide a direct fired downhole steam generator utilizing natural gas as a fuel and air as an oxidizing agent, which overcomes difficulties encountered in presently used units through use of a construction providing improved ignition and substantially increased life of the incorporated burner.
It is a further object of this invention to provide a high pressure combustor for a direct fired downhole steam generator, utilizing high pressure gaseous fuel and compressed air as an oxidizer, which is ignited through the controlled introduction of a pyrophoric material.
It is a further object of this invention to provide a method for effective and controlled introduction of a pyrophoric fluid for ignition of a high pressure combustor utilizing gaseous fuel and oxidizer.
It is a further object of this invention to provide a direct fired downhole steam generator having a high pressure combustor and utilizing gaseous fuel, oxygen as an oxidizer, and ignited by controlled introduction of a pyrophoric fluid.